A late evoked response (about 200 msec latency) occurs in monkey polysensory cortex following the initial response generated by electrical stimulation of primary somatosensory cortex or by peripheral input. This response is augmented by tetrahydrocannabinols, and often by low doses of LSD, mescaline and phenycyclidine, but is depressed by drugs like pentobarbital and chlorpromazine. The proposed work involves: a) an elucidation of the cells and pathways involved in this late response, by microelectrode and lesioning studies and b) a determination of the meaning to the animal of this late response by means of macroelectrode recording of evoked responses during operant behavior in monkeys with chronically implanted electrodes.